The present invention relates to a process for producing a pre-adherized polyester filament yarn having adhesion properties for rubber, to the polyester filament yarn made by that process and to impregnated tire cord made from pre-adherized polyester filament yarn.
Pre-adherizing methods for producing low-shrinkage polyester yarns which are high in tenacity and low in extensibility, even for industrial purposes, is known. For instance, WO 84/03707 describes a process in which suitable adhesion properties for rubber are provided by the application of a polyepoxide and of a tertiary amine as catalyst to polyester filament yarn after drawing. In this process, the amount of catalyst added is of considerable importance.
A combined process for simultaneous heat treatment and drawing of undrawn or fully drawn yarns is known from DE-A-2 056 707. This kind of process is possible at spinning speeds below 1000 m/min. At higher spinning speeds, as employed in the POY region, the pin and bobbin units are clogged so rapidly following application of a pre-adherizer as to make economical working impossible.
Existing methods solve the above mentioned problems for making tire cord only partly, since they do improve the adhesion properties of the polyester filament yarns for rubber but a tire cord produced therefrom is not a priori dimension stable. The dimension stability of tire cord, which is defined as the product of the breaking elongation or strength and the initial modulus divided by the thermal shrinkage, is an important property of polyester yarn as described in U.S. Pat. No. 5,045,260 issued to Remy Humbrecht, et al. A higher dimension stability indicates, for example, that tire cord made from the polyester yarn deforms less at high temperature and under various continued stresses.
The production of dimension stable polyester (PET) cord presupposes the spinning of preoriented, partially drawn yarn (POY). This can be accomplished by increasing the spinning speed. There have admittedly been proposals for keeping the spinning speed low by reducing the filament linear density, but this sharply increases the number of individual filaments making up the multifilament yarn, so that more filament breakages can occur in drawing. If, by contrast, the filament linear density is held at between 4.4 to 6 dtex/filament, adequate dimension stability in the tire cord can be achieved only if the preoriented, partially drawn yarn is spun at high speeds (&gt;2500 m/min).
Although residual drawability decreases with increasing spinning speed, the circumferential speeds of the drawing rolls and of the windup units increase continuously in the process. Consequently, the spin-drawing process has not only technical, for example speed related, but also economic, wear-related limits.
Drawing in a second operation separate from and at a slower speed than the spinning of the POY therefore represents an excellent alternative for producing dimension stable PET cords. However, in that case the application of the adhesion-improving substances only as the drawn yarn is being wound up must then be effected using a multiplicity of applicators, which makes the equipment complicated and uneconomical.